1. Field of the Invention
The present invention is related to adjusting optical energy values generated by devices on a sensor for dark current.
2. Background Art
Charge coupled device (CCDs) sensors and complementary metal oxide semiconductors (CMOS) sensors can be used to detect light over a range of many wavelengths in many diverse types of optical systems. The sensors are semiconductors that include an array of devices (called “pixels”) that absorb optical energy from impinging light and convert it to electrons, which are stored in a capacitor. The sensors generate optical energy values (e.g., electrons) proportional to the optical energy impinging on each pixel. Sensor pixels also generate dark current, which is an unwanted signal (e.g., noise) associated the sensor due to the presence of electrons generated by thermal and leakage effects. Dark current is temperature dependent, and can be reduced by decreasing the operating temperature of the sensor. Dark current is present and can also be localized in individual hot pixels. A value of the dark current at any one temperature will not be the same for each pixel in the array due to process variations. Dark current, as its name implies, adds an unwanted component raising the black level of the output signal. The net effect is to add an unwanted component to desired signal and this effect needs to be compensated for to generate accurate and optimal optical image values.
There are typically three types of CCD technologies: full frame, transfer frame, and interline. A basic description of these architectures can be found in CCD Primer MTD/PS-0218 Charged Coupled Device (CCD) Image Sensors, Eastman Kodak Company-Image Sensor Solutions, Revision No 1, May 29, 2001.
To accurately obtain a measure of the received optical energy, a dark current adjustment has to be made to the optical energy values. Conventionally, a calibration frame accomplishes this adjustment across the entire sensor before receiving the impinging light. The calibration frame is subsequently subtracted from the corresponding optical energy values generated based on the impinging light. However, the number of dark current generated electrons is proportional to exposure time and affected by temperature variations. Hence, the dark frame calibration optical energy values need to be taken using the same time duration as used by the measurement frame. Temperature fluctuations between sensing of the measurement frame optical energy values and the calibration frame adversely effect the desired adjustment. Thus, the sensor needs to be maintained at a constant temperature equal to that of the calibration frame. If this does not occur, the calibration frame cannot be accurately used to compensate for dark current.
This phenomenon becomes especially significant in systems that are used to measure very low light energy levels. These systems require long exposure times and thus accumulate a significant number of dark current generated electrons.
Typically, a calibration frame has to be taken often enough to ensure the compensation for dark current is accurate. This adds a time equal to the exposure time to the overall operation. Periodically stopping a lithography system, for example, to capture a calibration frame can be costly in terms of lost time and lost production, due to lower throughput. This is also true for any system used to capture optical measurements in low light energy levels.
Therefore, what is needed is a compensation method to compensate optical energy values for dark current without affecting throughput of a production system.